The overall goal of this project is to better understand how the adult colonic stem cell population responds to Aryl hydrocarbon receptor (AhR)-active toxicants in terms of stem cell-related phenotypes, energy metabolism, and the initiation and progression of colon cancer following exposure to AhR ligands in vivo. Adult somatic stem cells of the colon are of particular interest because they sustain self-renewal and are target cell for cancer initiating mutations. Perturbations in colon stem cell differentiation/plasticity, stem ell location/proliferation, and Wnt/beta-catenin signaling are generally believed to represent the earliest step towards colon tumorigenesis. Since the AhR and its ligands are known to affect hematopoietic stem cells and gastrointestinal biology, we propose to investigate the role of signaling through the AhR on colon stem cell dynamics and function. We will specifically focus on AhR-active toxicants, prototypical dietary AhR ligands and metabolites generated from dietary tryptophan by the microbiota (exogenous and endogenous ligands for the AhR, respectively). Since a majority of the AhR ligands are agonists and/or antagonists for the AhR in a tissue- and concentration- specific manner, we hypothesize that the combined agonist and antagonistic role of AhR ligands are important determinants in colon stem cell dynamics and responses. This is supported by our preliminary studies, which demonstrate for the first time that AhR ligands significantly influence colonic stem cell homeostasis and gene expression. The proposed experiments are novel and relevant because the impact of exposures to exogenous AhR ligands and their interactions with AhR-active intestinal endogenous AhR ligands on adult intestinal stem cell biology has not been determined. The following specific aims will be addressed: (1) Determine the agonist and/or antagonist activity of AhR-active toxicants and dietary AhR ligands on intestinal stem cell responses using colon cancer cell lines and an ex vivo organoid culture system; (2) Investigate the effect of microbiota-derived AhR ligands and their interactions with environmental toxicants and dietary ligands on intestinal stem cell responses in vivo and ex vivo; and (3) Quantify the number and spatio-temporal location of stem cells, DNA damage and targeted deletion in the colonic crypt at the initiation and progression stages of colon carcinogenesis following exposure to AhR ligands in vivo. Utilization of both in vivo and ex vivo models will allow us to dissect the effects of AhR ligands in the presence or absence of the AhR.